An expansion fastening device is usually used to fixedly connect multiple sheet workpieces, such as building panels, to one another, and includes a bolt and an expansion fastener. FIG. 1 shows a typical conventional expansion fastener 10, which includes a barrel body 11 having a plurality of longitudinally extended elongated slots 12 formed thereon to define a plurality of laterally spaced middle bars 13; a nut 14 connected to a head of the barrel body 11; and a bottom cover 15 connected to a bottom of the barrel body 11. As shown in FIG. 2, a bolt 16 can be extended through the bottom cover 15 and the barrel body 11 of the expansion fastener 10 to mesh with the nut 14. When the barrel body 11 is subjected to a pull, the middle bars 13 are brought to expand outward and become deformed. The deformed middle bars 13 and the bolt 16 together lock an attached object 17 to a supporting object 18.
The conventional expansion fastener 10 is disadvantageous in terms of assembling and installation thereof. First, the nut 14 and the bottom cover 15 are connected to the barrel body 11 by welding, which is time and labor consuming to result in high manufacturing cost of the expansion fastener 10. Second, two steps are required to install the conventional expansion fastener 10 on a sheet workpiece, such as a panel or a board. That is, a hole having size corresponding to the expansion fastener 10 must first be drilled on the supporting object 18; and then, the barrel body 11 is driven into the hole with a hammer, for example, so that the barrel body 11 is tightly fitted in the hole without the risk of rotating relative to the supporting object 18. The above installation is apparently troublesome and consumes a lot of time and labor. It is therefore desirable to overcome these disadvantages.